Radiation curing of automotive refinish compositions is becoming of increasing interest for use in body fillers, primers, surfacers and topcoats. The advantages of radiation curing are that it is quick, can be conducted at ambient temperature, and radiation-curable compositions can be formulated at high solids content, which is environmentally desirable. However, a problem in coating automobile bodies with radiation-curable compositions lies in the curing of areas not directly accessible to radiation such as shadow zones, for example, cavities, folds and other undercuts resulting from the automotive manufacturing process. Also, pigmented coating compositions may contain pigments that absorb radiation, such as carbon black and titanium dioxide, resulting in insufficient radiation to cure the resinous film-forming binder. Finally, ultraviolet radiation in the 200-400 nanometer wavelength range, although effective for curing thin coatings, is not particularly effective in curing the interior regions of thick coatings.
Therefore, it is desirable to have a composition that cures well when exposed to radiation and also cures via an alternate curing mechanism when radiation is insufficient to bring about complete cure.